Titanium or a titanium alloy is widely applied as a material for various parts of aircraft and machines and equipment for the chemical industry because of a high melting point (titanium has a melting point of 1,668.degree. C.), a high strength, a high toughness, a low density and an excellent corrosion resistance.
However, because of the high melting point of titanium or a titanium alloy as described above, it is not easy to manufacture various parts from titanium or a titanium alloy through a precision casting, which requires a high manufacturing cost.
A known method for manufacturing a titanium part at a lower cost is a powder metallurgy process which comprises: preparing a titanium powder, then forming the thus prepared titanium powder into a green compact of a prescribed shape through a press forming, and then sintering the thus formed green compact. Another known method for manufacturing a titanium alloy part at a lower cost is another powder metallurgy process which comprises: preparing a mixed powder by mixing a titanium powder with another metal powder which is to be alloyed with the titanium powder, then forming the thus prepared mixed powder into a green compact of a prescribed shape through a press forming, and then sintering the thus formed green compact.
When manufacturing various parts from titanium or a titanium alloy in accordance with one of the above-mentioned powder metallurgy processes, it is necessary to use a titanium powder or a titanium composite powder as a material.
As methods for manufacturing a titanium powder as the above-mentioned material, the following methods are known.
(A) First, a sponge titanium is prepared by means of any one of the following processes:
(i) A lumpy magnesium is charged into a steel vessel keeping an argon gas atmosphere, and heated to prepare a molten magnesium. Then, a liquid titanium tetrachloride at a room temperature is caused to fall dropwise from above into the vessel. The dropping titanium tetrachloride becomes a titanium tetrachloride gas because of the boiling point thereof of 136.degree. C. A sponge titanium (Ti) and magnesium chloride (MgCl.sub.2) are produced through a reducing reaction as expressed in the following formula (1) between the titanium tetrachloride gas and the molten magnesium: EQU TiCl.sub.4 +2Mg.fwdarw.Ti+2MgCl.sub.2 ( 1).
Then, the thus produced sponge titanium is separated from the magnesium chloride. The above-mentioned process for obtaining the sponge titanium is widely known as the "Kroll process".
(ii) A lumpy sodium is charged into a steel vessel keeping an argon gas atmosphere, and heated to prepare a molten sodium. Then, a liquid titanium tetrachloride at a room temperature is caused to fall dropwise from above into the vessel. The dropping titanium tetrachloride becomes a titanium tetrachloride gas because of the boiling point thereof of 136.degree. C. A sponge titanium (Ti) and sodium chloride (NaCl) are produced through a reducing reaction as expressed in the following formula (2) between the titanium tetrachloride gas and the molten sodium: EQU TiCl.sub.4 +4Na.fwdarw.Ti+4NaCl (2).
Then, the thus produced sponge titanium is separated from the sodium chloride. The above-mentioned process for obtaining the sponge titanium is widely known as the "Hunter process".
(B) Then, a titanium powder is manufactured by means of any one of the following processes with the use of the sponge titanium prepared as described above:
(a) The sponge titanium is pulverized by means of a grinding machine to manufacture a titanium powder (hereinafter referred to as the "prior art 1").
(b) The sponge titanium is first caused to absorb hydrogen to make the sponge titanium brittle. Then, the brittle sponge titanium is pulverized by means of a grinding machine to prepare titanium particles. The titanium particles are then dehydrogenated to manufacture a titanium powder (hereinafter referred to as the "prior art 2").
(c) The titanium powder obtained by the prior art 1 is formed into a green compact having an electrode-shape through a press forming. Then, the thus formed green compact is charged with electricity to melt same. The resultant melt is then cast into a high-purity titanium ingot. Then, the thus obtained titanium ingot is melted by means of an electric arc. The molten titanium is then caused to fall into a vessel keeping an inert gas atmosphere, and a compressed inert gas is ejected toward the falling flow of the molten titanium, or a centrifugal force is caused to act on the falling flow of the molten titanium, to atomize the molten titanium. The thus atomized molten titanium is rapidly cooled and solidified, thereby to manufacture a titanium powder (hereinafter referred to as the "prior art 3").
However, the above-mentioned prior arts 1 to 3 have the following problems:
(1) In the above-mentioned preparing processes (i) and (ii) of the sponge titanium, when a reducing reaction temperature in the steel vessel reaches at least 1,000.degree. C., iron forming the vessel reacts with produced titanium to produce Fe-Ti (Fe-Ti has a eutectic temperature of 1,080.degree. C.), resulting in a lower manufacturing yield of the sponge titanium. In order to avoid the production of the above-mentioned Fe-Ti, it is necessary to keep the reducing reaction temperature in the steel vessel to up to 960.degree. C. For this purpose, it is necessary to use a larger steel vessel, or to control the quantity of titanium tetrachloride supplied to the steel vessel. This control is not however easy. Even if a larger steel vessel is employed, there would not be much improvement in the productivity.
(2) In the prior arts 1 to 3, a sponge titanium is first prepared through reduction of titanium tetrachloride in accordance with the Kroll process or the Hunder process, and then the thus prepared sponge titanium is pulverized or atomized, thus requiring two steps, and hence requiring many facilities and much time. In addition, since the above-mentioned sponge titanium is prepared in a batch manner, the production efficiency is very low. Furthermore, each of the particles of the titanium powder manufactured through pulverization of the sponge titanium, having an irregular shape including a projection or an acute edge, is low in press-formability.
(3) In the prior art 3, it is necessary, as described above, to melt a high-purity titanium ingot, and then atomize the molten titanium, in order to manufacture a high-purity titanium powder. However, large-scale facilities are required for melting the titanium ingot and atomizing same.
(4) When manufacturing parts of a titanium alloy, uniform mixing of the titanium powder with another metal powder which is to be alloyed with the titanium powder, requires a high-level technology. It is therefore difficult to manufacture parts comprising a uniform titanium alloy.
Under such circumstances, there is a strong demand for the development of a method which permits continuous manufacture, in simple steps and at a high productivity, of a titanium powder or a titanium composite powder as a material for the manufacture of titanium articles or titanium alloy articles by a powder metallurgy process, but such a method has not as yet been proposed.